There are many types of image transduction devices which use arrays of individual transduction elements. Those devices include image scanners, cameras, artificial retinas, printers, and displays. Frequently, image transduction devices are designed around pixels. As used herein, a pixel is a fundamental building block of an image in that the image is represented by the composite states of all of its pixels. Since an image has both an optical representation (one that can be sensed, either directly or indirectly, based upon optical signals) and a data representation based upon electronic signals (such as voltage, current, or charge, either in digital or analog form), a pixel can be represented either optically or electronically.
In image transduction devices designed around pixels, each pixel usually corresponds to its own pixel cell which includes an image transduction element. Such transduction elements convert pixel image information between the pixel's optical and data forms. For example, a photodiode creates a certain number of separated electron-hole pairs per absorbed photon, and a liquid crystal-filled capacitor modulates incident light by an amount related to the charge on the capacitor plates. While there are many types of image transduction elements currently available, the more common ones are light emitting diodes, photodiodes, liquid crystal display elements, field emission tips and phosphors, and phototransistors. Some pixel cells may contain other electronics in addition to image transduction elements. For example, an array of liquid crystal display elements may have millions of individual pixel cells, each of which includes both an image transduction element comprised of a liquid crystal light valve and an associated transistor switch that facilitates active matrix addressing. Another example is an image scanner which may include thousands or even millions of individual pixel cells, each of which includes an image transduction element comprised of a light sensitive element, possibly a photodiode, and an associated transistor switch which controls the output of the photodiode.
Most large-area, image transduction arrays operate in real time Due to the large amount of data, optical and/or electronic, which is required to represent many images, and due to the speed required to process that data in a useful fashion, some image transduction devices could benefit by incorporating electronic memory within each pixel cell. Pixel cell based memory could simplify some types of image processing operations. Examples of such operations include pixel calibration, motion detection and compensation, adaptive filtering, and data compression and decompression, to name but a few. Furthermore, pixel cells with memory could reduce the number of expensive interconnecting lines per pixel and could facilitate the interchange of information required by some image processing algorithms.
However, because of deficiencies in the prior art, pixel cells with memory were not used, at least in large area pixel-based image transduction machines. Typically, all special functions were implemented in electronics external to the pixel cell. For example, in image scanners any variations in the gain and the offset of the pixel cell image transduction elements and any non-uniformity in the illumination source were usually corrected using external (non-pixel cell based) electronic circuits after the image is converted to digital form. Typical external electronic correction circuits include lookup tables and digital memory devices. Such external electronic correction circuits add to the cost and complexity of the scanner and create significant interface problems since the processing usually must occur in series rather than in parallel.
To compete with prior art image processing techniques, pixel cells with internal or integrated memory should fulfill a number of requirements. First, the memory should be compact enough to allow most of the pixel cell area to be dedicated to the pixel cell image transduction element. Second, power consumption should be low. Third, the memory should be nonvolatile so that the pixel memory information need not be continually refreshed, nor erased if power is removed. Fourth, the memory should have a well-defined memory state (a given input into the memory should result in a reliably known output from the memory.) Fifth, the state of the memory should affect the image transfer function of the pixel cell. The pixel cell image transfer function is defined to be the relationship between pixel cell inputs and outputs, whether those inputs and outputs are optical or electronic data.
Because analog memories can, in principle, meet the above requirements it would be highly desirable to integrate them into pixel cells that are used in large area image transduction devices. However, pixel cell analog memories have not been used in the prior art, at least partially because of problems with element-to-element variations in analog element fabrication and because of memory volatility. Therefore, pixel cells having integrated, nonvolatile analog memories coupled with methods that circumvent their problems would be useful.